It varies from horse to horse. Certain breeds mature faster than others, as do individuals. Everyone has a different opinion on the matter too that range from starting at 3 to starting at 6 or 7. It hasn't been proven that jumping early is going to ruin your horse in the long run, but excessive jumping, (or excessive anything for that matter) will cause wear and tear. I know some people who jump twice a week, but I also know other people who compete in the 3'6-4' range who school over fences twice a month tops during competition season.
It's really all subjective.